This invention relates to soft magnetic composites. More particularly, the invention relates to soft magnetic composites having improved strength. These composites which combine good soft magnetic properties with high strength are particularly useful as components in electrical machines.
Currently used components of soft magnetic composites prepared from pressure compacted coated iron powder have a relatively low compressive strength. This is due to the fact that these materials cannot be subjected to the usual method of improving the strength, i.e. sintering, since the high temperature required for sintering damages the insulating coating around the powder particles. Today soft magnetic composites are heat treated at a temperature below the sintering temperature in order to improve the magnetic characteristics. Also, the compressive strength of the component can be somewhat improved by such a heat treatment. WO95/29490 discloses a method of making a component having improved magnetic properties by compacting or die-pressing a powder composition of insulated particles of an atomised or sponge iron powder optionally in combination with a lubricant and in some cases a binder and subsequently subjecting the compacted composition to heat treatment in air at a temperature preferably not more than 50-500xc2x0 C. The strength of components prepared according to this patent is in the range 50-100 MPa, the higher strength being achieved at the cost of poorer magnetic properties. This strength is comparatively low and insufficient for certain applications.
Japanese Patent Publication 51-43007 discloses a method of manufacturing iron-based machine parts whereby an iron powder is pressure-compacted to obtain a green compact and the green compact is heated under an oxidising atmosphere including vapour at 400-700xc2x0 C. The purpose of this known method is to form iron oxide onto the surface of each iron grain. This procedure replaces the two steps involving dewaxing, i.e. the removal of lubricant, which usually is carried out at a temperature of at least 400xc2x0 C., and sintering, which is carried out at a temperature of at least 1100xc2x0 C. to form bonds between the metal particles. The Japanese publication also teaches that sizing of the body can be avoided because of the fact that the compacted and heat treated parts have high dimensional accuracy. The Japanese publication does not concern magnetic materials.
It has now been found that if uncoated iron powder particles, i.e. iron particles which are not provided with an insulating layer, are compacted and subsequently treated with vapour the strength of the material will increase but the energy loss in the material will be unacceptably large. When it comes to the coated iron powder particles used for magnetic applications it was found that the energy loss in coated material increases with increasing frequency and this tendency is even larger for vapour treated material than for coated material heated in air. During extensive studies it was however found that for frequencies less than 1000, preferably less than 300 Hz, it is possible to prepare soft magnetic composites having improved strength and a low energy loss.
The invention provides a process for the preparation of soft magnetic composites comprising the following steps: a) providing a low carbon powder of a soft magnetic material selected from the group consisting of an atomized or sponge powder of essentially pure iron or an iron-based prealloyed powder containing Si, Ni, Al or Co, b) providing the particles of the powder with an electrically insulating layer, c) compacting the powder to a composite body, and d) heating the composite body at a temperature between 400 to 700xc2x0 C. in the presence of vapor.
The invention also provides a composite body of compacted electrically insulated particles of a soft magnetic material, the compacted body having been heat treated in the presence of water vapour. The compacted body is useful in AC applications below 1000 Hz and preferably below 300 Hz.